Temperatures are measured in many industrial and commercial systems and processes to provide desired conditions in which the systems and processes operate. By accurately measuring the temperatures of the systems and industrial processes, associated equipment can operate at an optimum efficiency. Temperature measurements made for a combustion engine can be particularly critical, since the efficiency of the engine can be directly related to the temperature at which the engine operates.
Some applications for temperature sensing present a harsh or extreme environment which can affect the capability of a temperature sensor to accurately determine a temperature. For example, in some types of combustion engines, the temperature of the engine exhaust can provide an accurate indicator of the operating conditions of the engine. However, due to the harsh environment of the exhaust gas, the temperature sensing systems and the temperature sensors themselves, require robust packaging to prevent the extreme environment from affecting the temperature measurement. Highly complex packaging to protect the system or sensor can be required. Unfortunately, packaging of this type, while effective, can be costly or complex. In addition, additional electrical circuitry and the associated wiring may be needed to transmit the electrical temperature signals in an extreme environment. As a result, there are a relatively few currently available temperature sensors which can be used for such extreme applications.
Commercially available temperature sensors are known which measure temperatures at predetermined discrete periods of time, typically on the order of every few seconds or more. Such discrete measurement time periods can be too long for certain types of applications, and are therefore not suitable for fast, cycle-by-cycle temperature measurements. In particular, discrete time measurements, on the order of every few seconds, are not useful for control of advanced combustion strategies such as Homogeneous Charge Compression Ignition (HCCI) and Spark Assisted Compression Ignition (SACI) used in combustion engines.
Internal combustion engines compress and burn a quantity of an air/fuel mixture in each engine cylinder during successive engine cycles. If just enough air is present in order to burn all of the fuel, the air/fuel mixture is described as being a stoichiometric mixture. For customary internal combustion engines, the stoichiometric ratio of air to fuel is generally in a range of about 10-20 to 1 by weight depending on the type of fuel and other factors. If a greater proportion of air than for the stoichiometric mixture is present, the mixture is described as lean, and if a smaller proportion of air is present, the mixture is described as rich. An air-fuel equivalence ratio, lambda, is a normalized representation of the air/fuel mixture expressed as the ratio between an actual or measured air/fuel mix to the stoichiometric mix, and thus a stoichiometric mixture has a lambda equal to 1, a rich mixture has a lambda less than 1, and a lean mixture has a lambda greater than 1.
The lambda value of the air/fuel mixture burned in an engine cycle can impact the output torque and speed stability of the engine, a load-balancing between different engine cylinders, engine vibration levels, exhaust emissions produced by the engine, and the temperature of the exhausted gasses, as well as other factors. For example, a greater proportion of nitrogen oxide emissions are produced from combusting a lean mixture, and a greater proportion of other emissions such as carbon monoxide are produced from combusting a rich mixture. Thus, the operability, efficiency, and environmental impact of an engine can be improved by adequately controlling the lambda values of the air/fuel mixture supplied to the engine. However, the conversion from lean to stoichiometric to rich takes place over a narrow range of lambda values, which requires precise control of the lambda value.
U.S. Pat. No. 5,117,631 describes a method and apparatus for controlling the lambda value for the air/fuel mixture to be metered to an internal combustion engine. When a reference includes terms that are similar to terms used herein, the meaning of the terms as set forth herein controls. In U.S. Pat. No. 5,117,631, a control process includes alternating between increasing and decreasing the lambda value of the air/fuel mix so as to oscillate around a desired time-averaged value. However, since the lambda is controlled on a time-averaged basis, cycle-by-cycle variations may be present which can impact the characteristics of the engine.
Because the lambda value of a mixture directly relates to an oxygen quantity in the exhaust products of the combustion process, lambda control systems have been made which use lambda probes, or probes that sense an oxygen content of exhaust gasses produced by the engine. Combustion control systems have also been made which utilize thermocouple temperature sensors to sense the temperature of the exhaust gasses, since lean mixtures tend to burn hotter and rich mixtures cooler than stoichiometric mixtures. However, the time-constant for the response time of these types of sensors are generally too long to both take a temperature reading and adjust the lambda value of the mixture during a single engine cycle, since a single engine cycle may be on the order of milliseconds depending on the operating state of the engine.
U.S. Pat. No. 7,461,545 describes a method for monitoring cyclic variability in a reciprocating engine by analyzing exhaust gas temperature sensor signals. The method utilizes a modified thermocouple temperature sensor that is configured to have a reduced signal response time so as to enable cycle-by-cycle temperature readings of the exhaust gasses which can be fed to an engine control system. However, the resulting modified sensors may be expensive to produce and maintain, especially since the modified sensor is positioned directly in the harsh environment of the exhaust gas stream. Additionally, when the resulting measurements between various cycles differ by relatively small temperature values, adequate control of the engine may be difficult without complex filtering and analysis of the sensor values.
Other types of temperature sensors which have a shorter signal response time have been developed, such as bolometers, but such sensors are not optimized to withstand the harsh environment of the exhaust gas.
Therefore, there is a need for a fast temperature sensor that can withstand the harsh environment of an exhaust of an internal combustion engine and that is usable for cycle-by-cycle measurements of an exhaust gas, and for an engine control system that is optimized for cycle-by-cycle lambda control.